In connection with many aspects of man and machine interaction, it is important to track the motions of parts of a human body. For instance, in virtual reality ("VR") applications, the problem of making a fast, accurate, and economical head-tracker that operates throughout a large workspace is crucial. It is also important for other head-mounted display ("HMD") applications. Extensive research has been devoted to the development of optical, magnetic, acoustic and mechanical tracking systems, but head-trackers are still one of the weakest links in existing virtual-environment systems. The fastest and potentially most accurate trackers are mechanical, but these are typically clumsy and range-restrictive. The largest tracking range has been achieved at the University of North Carolina by optoelectronic methods, but this type of system is extremely expensive and difficult to install, calibrate, and maintain. This type of optical tracker is sometimes referred to as an "inside-out" tracker, because a camera that is mounted on the user is aimed out toward light sources mounted on the ceiling. Ultrasonic trackers are inexpensive, but must sacrifice speed to achieve reasonable range and are sensitive to acoustical interference, reflections, and obstructions. Magnetic trackers are the most popular because of their convenience of operation (they don't even require line of sight), but the maximum range is a few feet and distortions caused by metallic objects can be problematic. For reviews of the existing four head-tracker technologies, see: Meyer, K., Applewhite, H. and Biocca, F., "A survey of position trackers," Presence, 1(2): 173-200, Spring 1992; Ferrin, F., "Survey of helmet tracking technologies," SPIE, 1456, Large-Screen-Projection, Avionic, and Helmet-Mounted Displays: 86-94, 1991; and Bhatnagar, D., Position trackers for head mounted display systems: A survey, technical report, University of North Carolina at Chapel Hill, March 1993, all three of which are incorporated herein by reference.
All of the known trackers (magnetic, optical, mechanical and acoustical) require interaction with another component of the apparatus that is located a distance from the body being tracked. With magnetic trackers, a magnetic field generator is provided, spaced from the tracked body. With an optical or acoustical tracker, light or sound sources are provided at known locations. Mechanical trackers are connected to a reference through an arm-like device. Thus, none provide a self-contained apparatus for mounting on the body to be tracked, which apparatus can track the orientation of the body without interaction with radiation or energy from any other apparatus. Such a self contained tracking apparatus is desirable. As used herein, a "self-contained" tracking apparatus is one that can track the orientation of a body to which it is mounted, without interaction with radiation, energy, signals, or physical connections from any other apparatus.
Another drawback with acoustic and outside-in optical trackers is that they fundamentally only measure position. Orientation is then computed from the positions of three fixed points on the head. (By "orientation" it is meant herein the rotational alignment relative to an external reference frame.) Therefore, the angular resolution is limited by the uncertainty in the position measurements as well as the distance between the three fixed points on the head. With 100 mm spacing between the fixed points, a positional jitter of .+-.1.0-mm causes an orientational jitter of up to .+-.1.1.degree.. Additionally, since the position tracker is essentially part of the angular orientation tracker, it is not possible to meet independent specifications for the orientation tracker relative to the specifications for the position tracker.
It is also important to track other body members for other aspects of man and machine interaction. Most machines require a user instruction input device, typically actuated by the user's hand. The head, feet, torso and other body parts may also provide input instructions. Persons with special needs, such as paralysis of certain limbs, often use head and leg motions to complete tasks more typically conducted by hand motions.
Inertial navigation systems ("INS") using accelerometers and rate gyroscopes have been used for decades for ships, planes, missiles and spacecraft. Typically, such apparatus have been rather large, at least on the order of 8-10 cm in diameter and twice that in length, weighing on the order of 10 kg. An inertial navigation system is a type of self contained tracking apparatus, as that term is used herein. By "inertial apparatus", it is meant an apparatus that measures its own motion relative to an inertial reference frame through the measurement of acceleration.
A basic type of INS is called Strapdown INS, and consists of three orthogonal accelerometers and three orthogonal rate gyros fixed to the object being tracked. The orientation of the object is computed by jointly integrating the outputs of the rate gyros (or angular rate sensors), whose outputs are proportional to angular velocity about each axis. The position can then be computed by double integrating the outputs of the accelerometers using their known orientations. If the actual acceleration is .alpha. and the acceleration of gravity is g, then the acceleration measured by the triaxial accelerometers will be .alpha..sub.measured =.alpha.+g. To obtain the position it is necessary to know the direction and magnitude of g relative to the tracked object at all times in order to double integrate .alpha.=.alpha..sub.measured -g. Detailed information about inertial navigation systems is available in the literature, such as Broxmeyer, C., Inertial Navigation Systems, McGraw-Hill, New York, (1964); Parvin, R., Inertial Navigation, Van Nostrand, Princeton, N.J. (1962); and Britting, K., Inertial Navigation Systems Analysis, Wiley-Interscience, New York (1971), which are incorporated herein by reference.
A difficulty with using gyroscopes for head-orientation tracking is drift. Drift arises from integrating over time, a signal that is noisy, or has a bias. Drift would make the virtual world appear to gradually rotate about the user's head even when the user is not moving. By measuring the output of an angular rate sensor while it is at rest, it is possible to know its output bias and subtract the bias from all subsequent measurements. However, there is inevitably some random noise produced by the sensor in addition to its bias. In the short term, the angular drift is a random walk with RMS value growing proportional to .sqroot.t. However, the small bias that remains even in a well-calibrated system leads to a drift error that grows as t, which will eventually exceed the Brownian Motion error that grows as .sqroot.t.
U.S. Pat. No. 5,181,181, issued on Jan. 19, 1993, to Glynn, for "Computer Apparatus Input Device for Three-Dimensional Information," discloses a computer input mouse, which senses six degrees of motion using three accelerometers for sensing linear translation and three angular rate sensors for sensing angular rotation about three axes. The disclosure does not acknowledge or address the problem of drift.
Complete virtual environment systems also suffer from a problem that is not directly related to the problem of tracking body member motions and orientations. A great deal of graphical rendering is required to present to the user a visual image of the environment being simulated. The view to be presented depends on the user's orientation and position. The rendering requires significant computation, which is time consuming. Typically, the computation can not begin until the orientation is known. Thus, the speed of information acquisition is extremely important. It would also shorten the overall system latency if a reliable method of predicting the user's orientation in advance existed.